1. Field of Invention
This invention is related to intersecting traffic control, and in particular it relates to surface vehicular traffic control involving a plurality of crossing arteries with multidirectional propagation of traffic loads competing for passage through the intersections. The invention is intended for use with intersection signals which can be synchronized to a master clock by background cycle coordination.
Arterial traffic control system models exist which are suitable for applying load responsive control techniques to coordinate arterial traffic flow. The particular model of interest involves coordination of traffic signals based on a background cycle. A brief review of traffic engineering concepts and terminology is instructive to an understanding of the field of invention. Reference is made to Traffic Control Systems Handbook available from the United States Federal Highway Administration.
Fundamental to the understanding of the field of invention are the concepts of the multiple-phase intersection model, the phase, the split, the background cycle, and the offset. A multiple-phase intersection model is the mathematical model of the standard intersection. It comprises typically eight phases, a phase being an independently timed movement relative to an intersection. Specifically, a phase consists of an arterial crossing movement or a left turn movement which gives rise to potential conflicts. A typical eight-phase intersection model is illustrated in FIG. 1. FIG. 2 is a phase diagram, and FIG. 3 is a timing diagram.
Referring to FIGS. 1 and 2, the phase nomenclature of the eight-phase or quad intersection is defined. Phases One and Two are respectively a first left turn route and the opposing conflicting through route of the intersection. From those references, the left turn phases are numbered odd in a counter-clockwise manner and the through phases are numbered even in a counter-clockwise manner.
A background cycle (FIG. 3) is the quantum of time which must be set aside in a system of intersections for execution of a complete set of phases. In a background cycle coordinated traffic control system, the background cycle is at least as long as the shortest clock cycle to which all intersections in a group, i.e., synchronized set of intersections, can be synchronized.
Referring again to FIG. 3, a split is defined as the percentage of the background cycle during which a phase may be in green (allowed passage) or in yellow change (prepare to stop). In other words, a split is an allocation of the background cycle. There are generally two types of splits associated with each phase, the vehicle split and the pedestrian split. In each case, the split is the length of time allotted to the load (vehicle or pedestrian) to pass through the intersection. Background cycle coordination involves bunching the load into platoons and propagating the platoons through the intersection group with minimal interruption of flow.
Certain rules govern conflict resolution among phases. Reference is made to FIG. 2 which is a phase representation of the intersection model of FIG. 1. For example, only one phase in each one of the two sets comprising One through Four or Five through Eight may be active at one time. These sets are called the rings 1 and 2. A loop can be drawn around each ring. Where two active phases are present, both must be of or in transition to the phase set consisting of One, Two and Five, Six or the phase set consisting of Three, Four and Seven, Eight to assure conflict-free movement. These sets are considered to be on the same side of the "barrier" which divides the rings. Further, the total of all defined splits in each one of the rings 1 and 2 must equal one hundred percent of a fully serviced cycle. Further, the sum of the splits for the phases One and Two must equal the sum of the splits for the phases Five and Six, and similarly, the sum of the splits for the phases Three and Four must equal the sum of the splits for the phases Seven and Eight. Consequently, it is possible to analyze an intersection by merely examining one ring. These rules are sufficient to solve most intersection problems.
A further concept to be understood is the offset. The offset is the preselected time delay for nominal propagation of a platoon of vehicles between intersections in a group at a desired average vehicle speed. This concept is important to arterial background cycle coordination.
The primary method of arterial coordination of traffic flow is the background cycle in which there is specified a cycle length, multiple offsets between intersections, vehicle splits, pedestrian splits, phase sequence, and a synchronization of priority phases. What is needed is a traffic control system which not only can coordinate traffic flow but can respond to demand in individual phases without disrupting coordination.
2. Description of the Prior Art
Several traffic control systems are now in use or under development which are known to the art. The systems range from single intersection demand sensor systems which respond to presence of demand at each station (phase) of an intersection. Further systems are known to the art which are centralized decision-making systems. Examples are the central computer-based systems of Honeywell, Sperry Corporation, and the Federal Highway Administration Project known as the UTCS. These system employ sensors for volume and occupancy which convey demand information to a central station for processing. Absent a backup system mode, loss of telementry or central processor failure could mean loss of intersection coordination and control.
Traffic control systems have been under development at Multisonics, Inc. of Dublin, Calif., which distribute machine intelligence and machine decision-making responsibilities. For example, in the systems currently under development, autonomous Intersection Control Units (ICU) at each intersection are coupled in parallel to an autonomous Network Processing Unit (NPU), a set of which is coupled to a supervisory Master Processor (MP). The system is designed to minimize duplication of functions as well as to minimize data movement.